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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2018-474
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2018-474
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 26 Nov 2018

Research article | 26 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Carbon–Water Flux Coupling Under Progressive Drought

Sven Boese1, Martin Jung1, Nuno Carvalhais1,2, Adriaan J. Teuling3, and Markus Reichstein1 Sven Boese et al.
  • 1Max Planck Institute for Biogeochemistry, Hans-Knoell-Strasse 10, 07745 Jena, Germany
  • 2CENSE , Departamento de Ciências e Engenharia do Ambiente, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal
  • 3Wageningen University & Research, Hydrology and Quantitative Water Management, Droevendaalsesteeg 3, 6708 PB Wageningen, Netherlands

Abstract. Water-use efficiency, defined as the ratio of carbon assimilation over evapotranspiration (ET), is a key metric to assess ecosystem functioning in response to environmental conditions. It remains unclear which factors control this ratio during periods of extended water-limitation, and current semi-empirical water-use efficiency models fail to reproduce observed ET dynamics for these periods. Here, we use dry-down events occurring at eddy-covariance flux tower sites in the FLUXNET database as natural experiments to assess if and how decreasing soil-water availability modifies water-use efficiency on ecosystem scale. We demonstrate that an attenuating soil-water availability factor in junction with a previously discovered additive radiation term is necessary to accurately predict ET flux magnitudes and dry-down lengths of these water-limited periods. In an analysis of the attenuation, 20–33% of the observed decline in ET was due to the previously unconsidered soil-water availability effect. We conclude by noting the rates of ET decline differ significantly between FLUXNET sites with tall and short vegetation types and discuss the dependency of this rate on the variability of seasonal dryness.

Sven Boese et al.
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This study examines how limited water availability during droughts affects water-use efficiency. This metric describes how much carbon an ecosystem can assimilate for each unit of water lost by transpiration. We test how well different water-use efficiency models can capture the dynamics of transpiration decreasing due to increased soil-water limitation. Accounting for the interacting effects of radiation and water-limitation is necessary to accurately predict transpiration during these periods.
This study examines how limited water availability during droughts affects water-use efficiency....
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